The present invention is directed toward the field of electrical power supply management circuitry, and in particular, to improved power supplies for low duty-cycle radio frequency ("RF") communication systems, such as digital, packet-switched RF transmission devices. These types of systems typically include miniaturized low voltage power sources, and are characterized by relatively long time intervals between RF transmissions. These systems, however, require that large bursts of power be delivered quickly for transmitter operation. The present invention allows for substantial improvements in efficiency and effective battery life for such systems. In a conventional prior art digital RF communication system, the transmitter circuitry modulates a carrier signal with a binary signal, producing a transmitted RF sequence representing binary ones and zeroes. Under a packet-switched communications protocol, these binary one and zero bits are transmitted in discrete blocks ("packets") consisting of address, data, sender identification, and other control bits. The packets are not transmitted continuously, but are stored until a packet or group of packets is ready for transmission. Thus, the power amplifier for a conventional packet-switched transmitter requires high input power only for short intervals, with relatively long low power quiescent periods in-between. As a result, the "duty-cycle" of such a system, i.e., the percentage of the total cycle time taken by the active transmission time is quite low.
Despite the low duty-cycle of the system, the power amplifier for a conventional packet-switched transmitter produces a very high current load, drawing about 1000 milliamps (mA) or more for one second during transmission. In prior art technologies, the RF transmitter was often powered directly from a conventional battery; however, the high power demands of the power amplifier imposed severe limitations on the type of battery technology that could be used. Conventional carbon-based batteries typically could not provide sufficient instantaneous power for such a transmitter. While a conventional alkaline cell could have powered the transmitter, the equivalent resistance of such a cell will climb rapidly as the cell is depleted. This increasing resistance reduces the current that can be supplied to the transmitter and reduces the usable battery life. Ultimately, the instantaneous current supplied by an alkaline cell will fall below the power amplifier's input power requirements. In the prior art, this level of depletion would typically be reached while the battery still had significant capacity remaining; thus, a user would be required to replace a reasonably fresh battery.
Alternatively, some prior art packet-switched systems, such as portable RF modems, used internal, single-use alkaline cells to charge a rechargeable battery stack. This battery stack in turn supplied the power for the RF transmitter. These systems were quite inefficient, however, because the battery stack would be overcharged, storing up far more power than that normally needed for sending a short packet-switched message. Also, such power supply circuitry consumed space, was expensive, and was unnecessarily complex. In addition, conventional rechargeable batteries, such as a Ni--Cd cell stack, have a long charge cycle, sometimes measured in hours, and could withstand only a limited number of charge cycles, perhaps about 300, before such batteries themselves would have to be replaced.
Other types of batteries exist that provide high energy storage, but are incompatible with conventional packet-switched RF transmission systems because of the high equivalent series resistance ("ESR") of the cell. For example, a single-use lithium cell, such as an Ultralife.RTM. 9-volt cell (a registered trademark of Ultralife Batteries, Inc. of New York, N.Y.), has a very high stored energy rating of 9,000 milliwatt hours (mWhrs), as compared with only about 800 mWhrs for the typical rechargeable stack, or about 3000 mWhrs for a typical 9-volt alkaline cell. Unfortunately, a lithium cell has an ESR of over 10 ohms, even when new, and can only deliver a peak instantaneous power of about 0.75 watts. Because the typical RF transmitter requires 5 watts of input power and cannot tolerate an ESR of greater than 2 ohms, a cell such as the Ultralife.RTM. is not a viable power source despite its large storage capacity.
Similarly, a host computer auxiliary device power pin would be unsuitable to power a conventional packet-switched RF transmitter. Because most host computers can supply only about 0.75 watts to a PCMCIA slot or other types of card plug-in modules, such a source of supply could not directly power a typical packet-switched transmitter, whose power amplifier would require 5 watts of instantaneous power.
Therefore there remains a need for a power supply system capable of quickly delivering short bursts of high power with high efficiency, while remaining small enough for miniaturized RF communication applications.
There also remains a need for a power supply system capable of powering a low duty-cycle application requiring high instantaneous power from a battery, or other stored energy source having limited energy capacity, for substantial lengths of time.
There remains a further need for a self-contained, miniaturized, integrated power supply system capable of powering a low duty-cycle application requiring high instantaneous power from a current-limited source, such as a host computer auxiliary device power pin.
Finally, there remains a more particular need for a cost effective power supply circuit for a portable, PCMCIA-compatible radio modem, or a stand alone two-way pager system, which provides improved battery life, or alternately allows such a system to be powered directly from a host computer PCMCIA slot.